OP, I'd go with an mATX motherboard and case. I'm not a fan of the nmediapc case aesthetics. This is much cleaner looking. http://www.newegg.com/Product/Product.aspx?Item=N82E16811163174
If you could get by with an mITX and 2.5" HDD's, this Lian Li is probably one of the slickest HTPC cases I've seen. http://www.newegg.com/Product/Product.aspx?Item=N82E16811112357
As for CPU, you would be much better off with an Ivy Bridge CPU, if you're going to use the integrated graphics for video playback or QuickSync. Intel said the 24p playback issue is barely noticeably with Ivy Bridge integrated (source, paragraphs 4-5 http://www.bit-tech.net/hardware/cpus/2011/10/10/all-about-ivy-bridge/6
). Beyond that, if you want to do video encoding with QuickSync, you'll get something like 50% faster encoding speeds. And image quality is up (of course, gaming performance as well), if you ever did want to do some very lightweight gaming on integrated. That said, i5-3570K and a Z77 motherboard would be a decent combo. I'd wait for the i3's to release in a week or 2 if you wanted to save some money (there will be an i3 for ~$135 with HD 4000).
Quicksync benchmarks here - clearly it's the fastest. http://www.tomshardware.com/reviews/ivy-bridge-benchmark-core-i7-3770k,3181-7.html
For the RAM, I'd go with Samsung 8GB DDR3 1600 for the same price. No need for heatsinks on this RAM, 30nm so it uses less voltage and runs cooler, and if you wanted, you could easily overclock it past 2000mhz. Basically it's the best RAM anywhere near it's price. http://www.microcenter.com/single_product_results.phtml?product_id=0380720